The present invention relates to a method for fabricating semiconductor device and a semiconductor device.
Conventionally, a SOI (silicon on insulator) substrate that is a silicon substrate in which a monocrystal silicon layer is formed on a surface of an insulation layer has been known. By forming a device such as a transistor on a SOI substrate, parasitic capacitance can be reduced and insulation resistance can be increased. That is, high-integration and high-performance for a device can be achieved. The insulation layer is formed of, for example, a silicon oxide (SiO2) film.
To increase an operation speed of a device and further reduce parasitic capacitance of the device, the SOI substrate is preferably formed so that a monocrystal silicon layer has a small thickness. Then, conventionally, a method in which a silicon substrate is bonded to some other substrate such as a glass substrate and then part of the silicon substrate is removed by separation, thereby forming a SOI substrate, has been known (see, for example, Michel Bruel, “Smart-Cut: A New Silicon On Insulator Material Technology Based on Hydrogen Implantation and Wafer Bonding”, Jpn. J. Appl. Phys., Vol. 36 (1997), pp. 1636-1641).
Hereinafter, a method for forming a SOI substrate by bonding will be described with reference to FIG. 31. Among various techniques for reducing the thickness of a SOI layer, such as chemical polishing and a technique using porous silicon, a method using hydrogen implantation will be herein described. First, as shown in FIG. 28, a surface of a silicon substrate 101 serving as a first substrate is treated by oxidation, thereby forming a silicon oxide (SiO2) layer 102 serving as an insulation layer. Next, as shown in FIG. 29, ions of hydrogen, i.e., a peeling material are implanted into the silicon substrate 101 through the silicon oxide (SiO2) layer 102. Thus, a hydrogen-implanted layer 104 serving as a peeling layer is formed in part of the silicon substrate 101 located at a predetermined depth. Subsequently, RCA cleaning or like substrate surface cleaning is performed, and then, as shown in FIG. 30, a second substrate such as a glass substrate 103 is bonded to a surface of the silicon oxide layer 102. Thereafter, heat treatment is performed, so that a micro-clack is formed in part of the silicon substrate 101 located at the depth where hydrogen ions have been implanted. Thus, as shown in FIG. 31, part of the silicon substrate 101 is separated along the hydrogen-implanted layer 104. In this manner, the thickness of the silicon substrate 101 is reduced, thereby obtaining a silicon layer 101. After separation, the thickness of the silicon layer 101 is reduced to a desired thickness using polishing, etching or other various techniques as necessary. Also, using heat treatment or the like, crystal defects generated by hydrogen implantation is repaired and a silicon surface is smoothed.
In the above-described manner, the SiO2 layer (insulation layer) 102 is formed on the surface of the glass substrate (second substrate) 103 and the SOI substrate in which the silicon layer 101 is formed on the surface of the SiO2 layer 102 so as to have a small thickness is formed
Moreover, in general, it has been known that when a plurality of devices are formed on a substrate, in order to provide insulation adjacent ones of the plurality of devices from one another for isolation, for example, using a LOCOS (local oxidation of silicon) technique, a selective oxidation film (which will be herein referred to as a “LOCOS oxide film”) is formed. A general LOCOS oxide film is formed in the following manner. First, a silicon nitride film is formed over an oxide film formed on a surface of a silicon substrate and patterned. Then, oxidation is performed to the silicon substrate, so that an oxide film is selectively formed on a surface of part of the silicon substrate which is not covered by the silicon nitride film. Thus, a LOCOS oxide film is obtained. When forming a LOCOS oxide film, in the silicon substrate, silicon with a thickness corresponding to about 45% of the thickness of the LOCOS oxide film is consumed. As a result, a surface of the LOCOS oxide film becomes higher than a surface of part of the silicon substrate which is not oxidized by about half of the thickness of the LOCOS oxide film, so that level differences are generated.
The above-described structure including the LOCOS oxide film (which will be herein referred to as a “LOCOS isolation structure”) is formed on the silicon substrate 101, as shown in FIG. 32. A plurality of isolation regions and active regions are provided on a surface of the silicon substrate 101. In each of the isolation regions, a LOCOS oxide film 112 is formed and each of the active regions is located between adjacent ones of the isolation regions. In each of the active regions, for example, a gate electrode 111 of a MOS transistor is provided. On the other hand, a gate interconnect layer 113 is provided on the LOCOS oxide film 112 in each of the isolation regions. Furthermore, an insulation film 114 is formed over the gate electrode 111 and the gate interconnect layer 113 to flatten a surface of the entire substrate.